Internally screw threaded inserts for repairing damaged threads in holes such as spark plug sockets and for providing threads of relatively harder material in a socket of a softer parent body are widely known. Prior art devices for locking such inserts in place are subject to various disadvantages. For example, known devices are frequently of slender, and thus breakable, construction; they may also be of complex configuration thus being relatively expensive to manufacture and to use. Some known devices, when finally seated, occupy space which would otherwise be filled by the material of the insert itself. The absence of this insert material weakens the insert or its strength of connection to the parent material, thus requiring the use of a larger insert than would otherwise be necessary to achieve a desired level of strength in the final product.
Other devices of this type have utilized keys premounted on the insert, but these are also subject to breakage or to inadvertent displacement of the key prior to installation.
Still further prior art devices have utilized single keys inserted in preformed slots but this class of device is also subject to several difficulties. Many such keys are subject to inadvertent reverse installation which may destroy the insert or, at best, make the incorrectly installed lock inoperative. Many prior art patents suggest overcoming this difficulty by providing a dovetail slot configuration, but this is a difficult and expensive machining process.
Finally, still other devices are subject to difficulties of accurately inserting the key and providing adequate locking force, particularly in the case of relatively short inserts in which the maximum permissible key length may simply be inadequate to accomplish all of the required functions.